The present invention relates to a ceramic heater which can be used in the fabrication of a glow plug for a diesel engine, a heater for the air drawn into a diesel engine, an igniter for a gas or oil burner, a soldering bit, a heater for an electronic range or oven, and the like, and also to a method for manufacturing the same.
FIGS. 1 and 2 show a ceramic heater 3 which includes a heating coil 2' embedded in a ceramic insulator 1 having a circular cross section. If the heating coil had a circular cross section and were concentric with the insulator 1, as shown at 2 by broken lines in FIG. 2, the heater would provide a uniform temperature distribution in all directions radially of the insulator. Although the coil is formed with a circular cross section, it is difficult to ensure that it retains its circular shape, since during manufacture pressure is applied toward the axis of the coil in two diametrically opposed directions (as shown by arrows in FIG. 2) to compact a ceramic powder surrounding the coil in a mold by a hot press to form the ceramic insulator by sintering. The pressure so applied necessarily deforms the coil into an oval shape in cross section, as shown at 2' by solid lines in FIG. 2. The ceramic insulator 1 has a peripheral surface which is not uniformly spaced apart from the coil 2'. A temperature difference thus develops in the ceramic insulator and renders it liable to crack. No uniform temperature distribution can be obtained around the surface of the heater 3.
FIGS. 3 and 4 show a ceramic glow plug G adapted for use in a diesel engine for preheating the combustion cylinder or auxiliary combustion chamber thereof. The glow plug G includes a ceramic insulator 1 formed from heat conductive material and having a specially shaped cross section which is similar to a rectangle, as shown in FIG. 4. A U-shaped heating coil 2 is embedded in the insulator 1 and lies in a plane which is generally in parallel to the longer sides of the cross section of the insulator 1. When the glow plug G is installed in the combustion chamber of an engine as shown in FIG. 5, the heater 3 has its cross-sectional configuration disposed relative to the plane of FIG. 5, for example, as shown in FIG. 6 or in FIG. 7.
If the heater 3 faces in the wrong direction relative to a jet of fuel 15 from a nozzle 14, it will fail to preheat the fuel 15 properly, resulting particularly in the disadvantage that the engine cannot be easily started in cold temperature. This problem can be overcome if the heating coil is provided with a circular cross section, as shown at 16 in FIG. 8, and embedded concentrically in a ceramic insulator 17 having a circular cross section, as shown in FIG. 8. The difficulty in positioning the coil in a circular cross-sectional shape, however, has already been pointed out with reference to FIG. 2, and thus the coil is necessarily deformed into an oval shape as shown by solid lines at 16' in FIG. 8. The disadvantages of the ovally deformed coil have already been pointed out with reference to FIG. 2.